The spray painting of automobile bodies, truck engines, appliances, and other industrial goods is customarily carried out in enclosed areas called paint spray booths (PSB). These booths act to contain any fumes or oversprayed paint, to reduce the chances of air borne contamination and to protect the painters from these hazards. These booths vary in size, but are somewhat basic in their design and operation. A typical booth would thus consist of a work area, back section with water mist eliminators and a sump for collecting and holding the water for recirculation back to the mist eliminators.
The work pieces to be painted generally pass through the the work area while an air flow makes the oversprayed paint contact either the sump water or the spray from the water curtain. The air is scrubbed with recirculated water at the water curtain, passes through the mist eliminators and is removed by an exhaust fan. Roughly one-half of all paint sprayed ends up being captured by the water cycled through the mist eliminators.
Due to environmental pressure to reduce volatile organic compounds released into the atmosphere from the use of hydrocarbon solvent (oil) based paints, manufacturers are increasingly employing water-borne paints. The use of water-borne paints creates a problem in conventional PSB systems, since the over sprayed paint largely remains dispersed in the water held in the sump basin. Before this water can be recirculated to the mist eliminators for the removal of additional oversprayed paint, the dispersed paint previously collected must first be removed. Skimming or filtering the wastewater is ineffective since these paints exhibit no natural hydrophobicity or coagulation characteristics. Treatment programs for water-borne paints must therefore be different than treatment programs for hydrophobic oil based paints. The coagulation of dispersed paint particles is the primary objective in treating water borne paints. Tackiness is generally not a problem as it is with oversprayed oil based paints.
In some large industrial facilities, numerous paint spraying operations may be running simultaneously. Frequently, the water which has collected the oversprayed paint may feed into a single sump. At this location, the water is cleaned and then circulated back to the various spray booths from which it came. A unique problem arises in those industrial facilities in which water borne paints are sprayed in some booths, oil based paints in others and the water containing oversprayed paints from all booths is fed into a common sump. Effective treatment programs particles as well as detackify and coagulate the oil based paint particles. A treatment which may be effective for oil based paint particles may not have any effect on removing dissolved, hydrophilic water-borne paints. There is a need, therefore, to develop highly selective treatment programs to address these problems.